The present invention relates generally to semiconductor manufacturing processes, and more particularly, to methods and systems for controlling RF biasing in a process chamber.
Plasma processes are often improved by pulsing one or more parameters related to the plasma. By way of example, in a plasma etching system, a first RF source is used to form an inductively coupled RF plasma in a processing chamber. The RF plasma can then be used to etch a substrate. The substrate is typically placed on a substrate support in the processing chamber. A second RF source is often coupled to the substrate, through the substrate support, to create a negative voltage on the substrate.
Certain etching and other processes can be significantly improved by pulsing the negative voltage on the substrate at frequencies from between 1 Hz and 50 kHz. Typically, the peak voltage applied to the substrate is controlled via a filtered feedback loop. Applied bias voltages of 500V or higher are often used.
Further, some processes use a second level of pulsing in which pulsed bias is applied for time periods of a few seconds, removed and applied again, often in conjunction with other process parameters changing cyclically. This is referred to as mixed-mode pulsing (MMP). Unfortunately, the time periods of the mixed-mode pulsing are constantly being reduced to shorter and shorter time periods. The filtered feedback loop includes a time delay that can limit the accuracy and effectiveness of the feedback signal. There is a need for an improved feedback loop that minimizes the delay in the feedback loop and thus improve accuracy and effectiveness of the feedback signal and the switching of the mixed-mode pulsing.